A fibre-reinforced polymer (FRP) is a composite material comprising a polymer matrix reinforced with fibres. The fibers are usually glass, carbon, aramid or metallic fibres, such as steel fibres, while the matrix is usually an epoxy, vinylester, nylon or polyester thermosetting plastic. FRPs are typically organized in a laminate structure, such that each lamina contains an arrangement of unidirectional fibres or woven fibre fabrics embedded within a thin layer of light polymer matrix material. The fibres provide the strength and stiffness. The matrix binds and protects the fibers from damage and transfers the stresses between fibers.
FRP laminates have the ability to sustain a load without excessive deformation or failure, and because they respond linear-elastically to axial stress, i.e. when an FRP laminate is relieved of an applied axial tension it will return to its original shape or length. FRP laminates have a high strength to weight ratio, high creep resistance, a high modulus of elasticity (up to 450 GPa for example), high corrosion resistance, they can survive harsh environments and can be formed into complex shapes.
It is known that the benefits of an FRP laminate may be increased by pre-stressing the FRP laminate before bonding it to a structural member. An FRP laminate is namely pre-stressed and bonded to a structural member using an adhesive while maintaining the stressing force. The stressing force is released when the adhesive has hardened or cured. Pre-stressing the laminates before bonding them to structural members has several advantages. When bonding a pre-stressed FRP laminate to a concrete structure these advantages include:                a reduction in deformations due to live loads and thus performance enhancement in the serviceability limit state,        crack width reduction on the tensile part of the structure and consequently an increase in durability        the provision of a negative moment against dead loads and more capacity for live loads, and        a compensation for the lost pre-stress in a pre-stressed concrete structure (due to the corrosion or damage of tendons for example).        
When bonding an FRP laminate to a steel structure the advantages include the enhancement of the fatigue strength of the steel structure and the prevention of fatigue crack formation or propagation in the steel structure.
A problem when using bonded pre-stressed FRP laminates when repairing or strengthening a structural member is that high shear stresses may build up at the ends of FRP laminate in the adhesive layer that bonds the FRP laminate to the structural member. These shear stresses are normally several times higher than the strength of conventional adhesives, such as epoxy resins, that are used to bond the FRP laminate to the structural member. Shear stresses of 100-150 MPa can for example arise at the ends of an FRP laminate, whereas conventional adhesives can withstand only shear stresses of 20-25 MPa. The shear stresses may give rise to delamination or debonding of the FRP laminate from the structural member, whereby the delaminating or de-bonding may be initiated at the ends of the FRP laminate and propagates inwards from the ends of the FRP laminates. De-bonding limits the capacity of the strengthening system below its ultimate flexural capacity and this failure mode can be characterized by a sudden separation of the FRP laminate from the structural member rather than by the ultimate flexural capacity of the cross section of the strengthened structure.
Mechanical anchors are usually used to solve the problem of high shear stresses at the FRP laminate ends. However, there are several problems associated with using a mechanical anchoring system. Mechanical anchors are in many cases rather complicated, time-consuming and costly to manufacture, install and inspect. They often need to be manufactured with very close dimensional tolerances for the specific structural member to be strengthened. The structural member on which they are mounted often needs to be modified (a part of the structural member may need to be cut out and removed and bolts may have to be drilled into the structural member and fixed in place using adhesive or mortar bonding for example). The mechanical anchors may be susceptible to moisture and dust accumulation which may result in the corrosion of the anchoring system. Furthermore, galvanic corrosion may take place when metal anchors are used to repair or strengthen a structure comprising a dissimilar metal. Additionally, the drilling of steel structures to install the mechanical anchors is inevitable. In some cases, where the aim of using pre-stressed laminates is fatigue strength enhancement, drilling holes in a structure which are normally situated in a high moment area, could cause new fatigue-prone points in the structure.
U.S. Pat. No. 6,464,811 discloses a method of reinforcing a construction part with lamellar, fibre-reinforced plastic strips. The lamellar strips are pre-tensed with a tensioning device, treated with adhesive in a pre-tensed state and then moved to the construction part to be treated together with a tension device. The tension device is provisionally fixed to the construction part with displaceable fixing devices. Thereafter, the lamellar strips are pressed against the construction by means of an air bag or air hose until the adhesive has hardened. This patent discloses that the strips may be pre-stressed by different amounts by pre-tensing a first part of the strip using a first tension and adhering that first part of the strip to the construction part, and then, once the adhesive has cured, pre-tensing a second part of the strip using a second tension and then adhering that second part of the strip to the construction part. This method is however quite time consuming and complex, especially if long strip lengths are used, and, if an existing structure, such as a bridge, is being reinforced; it could be out of service for a considerable period of time.